稀土鈣鋇銅氧化物超導體之高壓合成與特性研究

Abstract

本研究以高溫高壓法，於990℃、26∼30bar氧壓下，合成(R1-xCax)2Ba4Cu7O15-δ(簡稱R247; R=Y, Dy, Sm)等三系列高溫超導體，以探討鈣離子之取代對上述氧化物之晶體結構化學、超導臨界溫度(Tc)、電阻率、磁化率以及氧含量變化之效應。 X光繞射數據顯示：當R247的R為Y、Dy、Sm時，鈣離子取代上限(x值)分別為0.2、0.15、與0.10。若x值超越取代上限，則R247不再穩定而轉變為(Y,Ca)Ba2Cu3O7-δ相。此外，摻加鈣離子同一系列之R247相之晶格常數與正交扭曲值，則隨鈣離子含量增加，晶格常數因鈣離子可能取代R或Ba位置，而呈現無明顯變化。正交扭曲值則呈現下降趨勢或無明顯之變化。 另一方面，當R為Y、Dy、Sm時，Tc隨鈣離子含量增加而由93.8K、83.4K與64.5K分別逐漸遞減或遞增至83.8K(x=0.20)、83.3K(x=0.15)、與75.0K(x=0.10)。而常態電阻率與超導體積分率，前者隨摻加鈣離子含量增加而上昇，而後者則呈現下降之趨勢。而三個摻鈣的R247系列，氧含量則隨鈣離子上昇而呈下降。 本研究發現在三種摻鈣的R247超導體系列中，Tc之變化應同時取決於結構化學與電洞濃度等兩關鍵因素。

Three series of (R1-xCax)2Ba4Cu7O15-δ (R247; R=Y, Dy, Sm)cuprate superconducors have been synthesized at 990℃ and under 26-30 bar high oxygen pressure. The effect of Ca2+ doping variation of oxygen content, structural chemistry, and superconductivity was investigated. The substitution limit of Ca2+ for R in R247 was determined to be 0.20, 0.15, and 0.10 for phases with R=Y, Dy, Sm, respectively, as indicated by x-ray diffraction data. In stead of R247, phases such as RBa2Cu3O7-δ and other impurities was found to form when the substitution limit was exceeded. In addition, with increasing Ca2+ content lattice parameters of Ca-doped R247 phases were found to vary insignificantly, whereas the orthorhombicity was found to increase or vary insignificantly. The former observations were attributed to the possibility of dual substitution of Ca2+ for R3+ or Ba2+ sites. On the other hand, superconducting temperature (Tc) of R247 phases was found to decrease or increase from 93.8 K, 83.4 K, 64.5 K to 83.8 K (x=0.20), 83.3 K (x=0.15), and 75.0 K (x=0.10) for phases with R=Y, Dy, Sm, respectively. With increasing Ca2+ content normal state resistivity of Ca-doped R247 was found to increase while volume fraction of superconductor was found to decrease. In general, oxygen contents of Ca- doped R247 phases were found to decrease with increasing Ca2+ content as indicated by iodometric titration. Variation of Tc for the three series of Ca- doped R247 was discovered to be highly dependent upon critical factors such as structural chemistry and hole concentration which was found to be difficult to control in this study.